The present invention relates generally to image compression coding technology and code division multiple access (CDMA) mobile communication technology, and more particularly to an implementation method for image compression coding and transmission which is adequate for the soft capacity characteristic of CDMA system.
In image compression technology, if it is allowed a certain extent of distortion, then an image can be compressed in a certain extent. At the same time, considering each coded image frame, as their characteristics are different, so the compression ratio needs to be different. For example, images with different entropy may have different compression ratio; the one with larger entropy may have more compression and vise versa. Another example, for moving image, if the changes are slow, then the compression ratio can be higher than normal; this can be got by decreasing the frame rate of image coding. Therefore, in order to guarantee the quality of image transmission, different coded frame should have different compression ratio, i.e. the image coding rate should be different accordingly.
The CDMA digital mobile communication system has great potential power. In the CDMA system, system capacity is a soft capacity concept. For example, the system manager may raise the frame error rate to increase the available channels during peak hours of telephone traffic. Again, the CDMA system is a self-interference system, when its neighbor cells have less load, interference sent to the cell is smaller, so the capacity can be increased adequately. The soft capacity characteristic can be represented such as cell breath function, etc.
During the image transmission in CDMA system, the algorithm of digital image coding is a constant rate algorithm controlled by an external command, such as CCITT H.261 algorithm. This kind of algorithm controlled by an external command is impossible to consider the characteristics of the image itself, because the coding rate is a constant. Under maintaining the quality of image transmission greater than a certain threshold, it is limited raising of the CDMA system capacity in statistic sense. This is because maintaining image transmission quality, i.e. keeping the channel transmission quality unchanged, there is a certain proportional relationship between image coding rate with emission power. Changing the coding rate corresponds adjusting the emission power, so it is possible to increase the system capacity in statistic sense. As said above, the constant coding rate algorithm, controlled by external command, can not consider the characteristics of the image, this makes the soft capacity characteristic of the CDMA system has not been used greatly.
For constant image coding rate algorithm cannot be adequate of the soft capacity characteristic of CDMA system, the present invention provides an image transmission method which change image coding rate according to the characteristics of an image, in order to increase system capacity in statistic sense.
The invention proposes an image transmission method, which maintains the image transmission quality and increases the capacity of CDMA mobile communication system; the method comprises the steps of:
(a) according to the characteristics of an image, make image coding with changing rate;
(b) maintaining the channel transmission quality, change the spread spectrum factor and adjust the emission power accordingly, or use the discontinuous emission mode without changing the spread spectrum to increase the system capacity.
The invention also proposes an image transmission method, which maintains the image transmission quality and increases the capacity of CDMA mobile communication system; the method comprises the steps of:
(1) according to the characteristics of an image, make image coding with changing rate;
(2) according to the image coding rate, calculate spread spectrum factor;
(3) according to the spread spectrum factor, change emission power;
(4) transmit with the spread spectrum;
(5) repeat steps (1) to (5) for the subsequent frame.
The invention still proposes an image transmission method, which maintains the image transmission quality and increases the capacity of CDMA mobile communication system; the method comprises the steps of:
(1) according to the characteristics of an image, make image coding with changing rate;
(2) according to the image coding rate, calculate interrupted point;
(3) for interrupted transmitting signal, randomize and rearrange it within the frame with interleave algorithm;
(4) transmit with spread spectrum;
(5) repeat steps (1) to (5) for the subsequent frame.
The said image coding with changing rate may apply the subband coding with threshold method or the differential predictive coding method.
The said subband coding with threshold comprises at least the following steps:
(1) divide the original image into different characteristic images;
(2) according to the human vision and statistic result, set a threshold;
(3) make image coding, the data less than the threshold is set to zero;
(4) quantize the non-zero value and make compression coding.
The divided images may further be coded by the subband coding with threshold again.
In the said subband coding with threshold method, by increasing the threshold value, the image coding rate can be decreased to increase system capacity.
The said differential predictive coding method comprises at least the following steps:
(1) calculate the difference of two timely successive frames;
(2) set the threshold, according to the human vision and statistic result;
(3) use the algorithm with low compression ratio when the calculated result of the differences in step (1) is greater than the threshold;
(4) use the algorithm with high compression ratio when the calculated result of the differences in step (1) is smaller than the threshold.
As CDMA system is a self-interference system, its capacity is a soft capacity in concept, and there is a proportional relationship between its coding rate and emission power. When changing the coding rate and emission power, the system capacity is increased accordingly. Therefore, maintaining the image transmission quality, if the compression ratio of coding rate and coding rate are changed according to the specific characteristics of the image, then the compression ratio of the image will be further increased. In this way, the soft capacity of the CDMA system is possibly increased in statistic sense. As the present algorithm, controlled by external command does not take account of the image characteristics, so it does not utilize the soft capacity characteristic of the CDMA system greatly.
Using the image compression code transmission mode with changing coding rate of the invention, the stability of an image can be maintained and the capacity of the CDMA system will be increased in statistic sense.